The present invention is directed to a prosthetic cup assembly that is disclosed in the context of a hip prosthesis and more particularly to such an assembly that permits the prosthesis to be implanted at a desirable fixation point in the hip while allowing the face of the bearing surface to be oriented independently of the implant position within the acetabulum.
It is known to provide an acetabular cup assembly that includes a metal shell component for attachment to an acetabulum to replace the natural socket and a bearing component (commonly made of plastic, metal or ceramic) that is inserted into the shell to provide a bearing surface for receiving a femur ball prosthesis element or the proximal end of the femur head. See for example, Englehart et al., U.S. Pat. No. 5,049,158, the disclosure of which is expressly incorporated herein by reference. In addition, traditional bearing components include a built-up lip around a portion of the bearing surface. See for example, U.S. Pat. Nos. 5,282,864 and 5,413,603 to Noiles et al., the disclosures of which are also expressly incorporated herein by reference.
A problem that can occur with such acetabular cup assemblies is that they are configured so that the shell needs to be implanted in a very specific location in the acetabulum in order for the bearing surface to present a desirable surface for the head of the femur ball prosthesis to ride against in order to simulate the patient's natural anatomical structure and range of motion.
Some acetabular cup assembly designs have used a third member as a way to maintain macrostability of the assembly parts while maintaining dome loading. Dome loading designs essentially ensure contact in the dome region by leaving clearance under the lip of the liner. These dome loading designs however, cause the insert to seat in the direction of the applied load.
The shape and orientation of the concave face of the acetabular shell and the shape of the convex mating surface of the bearing received therein establish the orientation of the bearing face of the bearing when it is received in the acetabular shell. Once an acetabular shell is fixed in place, the orientation of the concave face of the acetabular shell, and therefore the bearing face of the bearing, can not be changed without disrupting the bone shell interface. Face changing poly liners do offer some measure of adjustment, but this option is not available for all bearings. The bearing face position is largely dictated by the position of the shell since they typically cannot be positioned independent of one another.
Two currently available ways to change the bearing face orientation of the bearing is to use a face changing bearing (if available) or remove the shell thereby disrupting the shell bone interface, and repositioning the shell. Face changing liners do not always offer enough change.